of Edinburgh , Session 1885-86. 
879 
two basins, 20 fathoms deep, which are separated by a bank that 
is only partially exposed at “low water springs.” H.M.S. “Lark” 
found a sheltered anchorage in the western basin. The bottom in 
the deeper parts is formed of a chalky mud. In the interior of the 
lagoon, corals thrive most on the banks that face the breaks in the 
reef and on the sides of the entrances. There are, however, extensive 
sandy tracts covered by less than a fathom of water at low tide, on 
which may be observed very large flat-topped masses of Porites, 
Oima Atoll , adapted from a Sketch of my own , assisted by the Plan 
in the Admiralty Chart . 
SCALE OF MILES 
o f i Hz 
The white portion represents the reef above the sea level bearing the six 
volcanic islets. The inner shaded space is the lagoon, of which the 
lighter tint represents depths less than a fathom, and the darker tint the 
deeper part, which has a depth of 20 fathoms. 
10 to 16 feet across, completely dead, and only projecting a few 
inches above the sand. These huge corals had evidently been killed 
by the accumulation of sand ; and it was apparent that through 
this agency the lagoon was filling up. This atoll does not seem to 
have experienced any upheaval since the commencement of its 
growth, although it is situated in an area of elevation. However, 
the existence of old erosion lines, with their accompanying caverns, 
on two of the volcanic islets affords evidence of a prior upheaval 
\ to the extent of some four or five feet * I should observe that 
numerous full-grown Tridacnce {T. gig as) , which occur alive in the 
* The bearing of this fact is referred to on page 867. 
